The compound 1-(4-amino-6,7-dimethoxy-2-quinazolinyl)-4-(tetrahydro-2furoyl)piperazine is known by the common name "terazosin" and is the subject of U.S. Pat. No. 4,026,894. This patent discloses in Example VI a process for making the anhydrous crystalline polymorph of terrazosin designated "form I" throughout this application. The compound is known to have utility for the treatment of hypertension and benign prostatic hyperplasia, and pharmaceutical compositions comprising terazosin and its salts are claimed in U.S. Pat. No. 4,112,097. The dihydrate crystalline form of the hydrochloride salt of terazosin is disclosed and claimed in U.S. Pat. No. 4,251,532, and is marketed under the trade name Hytrin.RTM.. The R(+)-enantiomer of terazosin is disclosed and claimed in U.S. Pat. No. 5,212,176.